As is known, knitting machines comprise a needle holder that supports a plurality of needles, which are arranged mutually side by side and can be actuated with an alternating motion along their axis with respect to the needle holder in order to form knitting. In this alternating motion, the needles are conveniently guided by the needle holder and/or by other elements connected to the needle holder.
More particularly, each needle can be actuated with an alternating motion along its own axis with an extraction motion, by means of which the needle is extracted with its tip or head and with a portion of its shank from one end of the needle holder in order to drop, onto its shank, the loop of knitting formed previously and/or in order to pick up the yarn or yarns dispensed at a drop or feed of the machine, and with a retraction motion, by means of which the needle is made to retract with its tip into the end of the needle holder in order to form a new loop of knitting, performing knockover of the loop of knitting formed previously.
In their alternating motion with respect to the needle holder, the needles are guided, being arranged within sliding channels defined within the needle holder and optionally in other elements connected to the needle holder.
For example, in single-cylinder circular knitting machines, in which the needle holder is constituted by a cylinder, known indeed as needle cylinder, which has a vertical axis, the needles are arranged slidingly within axial sliding channels defined on the lateral surface of the needle cylinder. These sliding channels are separated from each other by elements known as slats. In some cases, the sliding channels are constituted by axial grooves which are defined in the lateral surface of the body of the needle cylinder. In these cases, the slats are constituted by the portions of the needle cylinder that are located between the various axial grooves. In other cases, the sliding channels are obtained by inserting laminas, which constitute the above cited slats, within axial cuts defined in the lateral surface of the needle cylinder.
Single-cylinder circular knitting machines are generally provided, at the upper end of the needle cylinder, with an annular element, which is fixed integrally around the upper end of the needle cylinder and is provided with radial cuts, inside each of which a sinker is arranged, and these radial cuts are angularly offset around the needle cylinder axis with respect to the needle sliding channels so that each sinker is located between two contiguous needles.
The sinkers can move radially with respect to the needle cylinder so as to cooperate with the needles in forming the knitting. These sinkers have an upper side, known as knockover plane, on which the portion of knitting located between two contiguous needles rests when said needles, after picking up the yarn or yarns at a feed or drop of the machine, retract with their tip into the end of the needle cylinder in order to form a new loop of knitting. In these machines, proximate to the end of the needle cylinder from which they exit in order to pick up the yarn or yarns, the needles are guided within knitting forming channels, each delimited laterally by two contiguous sinkers. These knitting forming channels in practice constitute extensions of the sliding channels cited above.
Circular knitting machines with cylinder and dial are composed of a needle cylinder, which is substantially provided like the needle cylinder of single-cylinder machines and with a dial which is arranged above and coaxially with respect to the needle cylinder. In the upper face of the dial a plurality of sliding channels is provided, which are oriented radially with respect to the axis of the dial and are angularly offset with respect to the sliding channels defined in the needle cylinder. A needle is arranged in each one of these sliding channels of the dial and can be actuated with an alternating motion along the corresponding sliding channel so as to exit with its tip from the peripheral edge of the dial in order to pick up the yarn or yarns provided at a drop or feed of the machine and so as to retract in order to form a new loop of knitting, in a manner which is similar to what has been described with reference to the needles of the needle cylinder. These machines have no sinkers and the needles arranged in the dial cooperate with the needles arranged in the needle cylinder in forming the knitting. In particular, the needles of the dial can be used to retain the knitting, formed previously, during the extraction motion of the needles located in the needle cylinder, preventing it from being drawn upwardly, causing knitting errors. Likewise, the needles of the needle cylinder can be used to retain the knitting, formed previously, during the motion of extraction of the needles arranged in the dial.
Some types of single-cylinder circular machine, particularly with a high gauge, have no sinkers and the function of retaining the knitting during the needle extraction motion is performed by retention elements which are arranged laterally to the needles inside the needle cylinder and protrude, with one of their ends, above the upper end of the needle cylinder. These retention elements are shaped like laminas and are laterally adjacent to the needles, providing a sort of comb that can engage the loops of knitting formed previously, preventing them from being drawn upwardly by the rising motion of the needles during their extraction motion. Retention elements of this type are disclosed for example in WO2008/003463 and WO2008/145433 by the same Applicant.
In machines without sinkers, the knitting forming channels are constituted by an end portion of the needle sliding channels. In some machines, the knitting forming channels are delimited laterally by laminar elements, known as secondary sinkers, which are driven into axial cuts defined in the lateral surface of the needle cylinder proximate to its upper end.
In any case, in conventional machines, both in single-cylinder circular machines and in circular machines with cylinder and dial or more generally with a double bed, in each needle holder, be it a cylinder or a dial, there are as many sliding channels as there are knitting forming channels, and each one accommodates a needle.
The actuation of the needles with an alternating motion along the corresponding sliding channel is obtained by providing, for each needle, at least one heel that protrudes from one side of the needle holder and can engage paths defined by cams that face said side of the needle holder. The shape of these paths, together with the fact that the needle holder is moved with respect to the cams along a direction that is transverse to the extension of the sliding channels, achieves the alternating motion of the needles along the corresponding sliding channel. The engagement of each needle with these cams leads to a lateral thrust of the needle, i.e., transversely to the extension of the sliding channel in which it is arranged. This lateral thrust is discharged onto a side of the sliding channel, i.e., onto the slat that delimits on one side said sliding channel and must have an adequate thickness in order to withstand said thrust.
In order to calculate the thickness of the slat, i.e., the thickness of the region of the needle holder that separates two contiguous sliding channels, in conventional machines it is possible to apply the following formula:Slat thickness=(25.4/E)−(S+G)where:    E=gauge (needles/inch)    S=needle thickness (mm)    G=needle play (mm)
Needle play is the play between the needle and the sliding channel in which the needle is accommodated in a direction which is transverse to the axis of said needle.
As can be seen from the above cited formula, the thickness of the slat decreases as the gauge increases and the possibility to increase the gauge has a limit which is linked to the mechanical rigidity of the slat, whose strength, for an equal material used, obviously decreases as its thickness decreases.
In recent years, the market of knitting machines has seen a significant growth in the demand for machines that have ever higher gauges due to greater demand for increasingly fine and light knitted fabrics.
This demand has conferred an increasing importance to the problem of being able to provide machines with ever higher gauges. However, this problem cannot be solved easily, due to the fact that the thickness of the needle cannot be reduced beyond a certain limit, since it must meet specific requirements dictated by textile parameters and is already low in current machines with high gauges and the play necessary for its operation also cannot be eliminated.
On the other hand, the thickness of the slat also cannot be reduced excessively, since the slat is assigned the task of contrasting the lateral thrust that derives from the engagement of the needle with the cams that cause its actuation with an alternating motion within the corresponding sliding channel.